Privacy systems are in use both in telephone and other types of communication systems. Such privacy systems render signals unintelligible to avoid interception by unauthorized listeners and, in many cases, are restricted to selected communication channels over which secret messages are sent. However, security arrangements are also frequently appropriate when messages are transmitted over common communication paths easily accessible to third parties, e.g., the microwave links in a long-distance telephone network or the time-multiplexed lines of a time division system.
One known privacy communication arrangement, disclosed in U.S. Pat. No. 4,100,374 of N. S. Jayant et produce an uncorrelated scrambled signal. The process involves sampling the information signal at a predetermined rate and dividing the samples into groups of N successive samples. To encrypt the signal, each successive sample group is permuted by transposing the samples within the group. The Jayant et al. system is, however, vulnerable to a "code-breaking" process whereby the scrambled signal is recorded and then analyzed by a computer to determine the scrambling technique involved. The original information signal is then recreated by performing the inverse of the determined technique on the recorded signal, thus effectively "code-breaking" the system.
A recognized problem in the art is the vulnerability of known privacy systems including the Jayant et al. system, to "code-breaking", particularly where all the information in the original signal is present in the encrypted signal and where the encrypted signal as in the Jayant et al. system, can be recorded by unauthorized listeners and subjected to an exhaustive analysis by computer to defeat the encryption.